MicroRNAs (miRNAs) are a small non-coding family of 19-25 nucleotide RNAs that regulate gene expression by targeting messenger RNAs (mRNA) in a sequence specific manner, inducing translational repression or mRNA degradation depending on the degree of complementarity between miRNAs and their targets (Bartel, D. P. (2004) Cell 116, 281-297; Ambros, V. (2004) Nature 431, 350-355). Many miRNAs are conserved in sequence between distantly related organisms, suggesting that these molecules participate in essential processes. Indeed, miRNAs are involved in the regulation of gene expression during development (Xu, P., et al. (2003) Curr. Biol. 13, 790-795), cell proliferation (Xu, P., et al. (2003) Curr. Biol. 13, 790-795), apoptosis (Cheng, A. M., et al. (2005) Nucl. Acids Res. 33, 1290-1297), glucose metabolism (Poy, M. N., et al. (2004) Nature 432, 226-230), stress resistance (Dresios, J., et al. (2005) Proc. Natl. Acad. Sci. USA 102, 1865-1870) and cancer (Calin, G. A, et al. (2002) Proc. Natl. Acad. Sci. USA 99, 1554-15529; Calin, G. A., et al. (2004) Proc. Natl. Acad. Sci. USA 101, 11755-11760; He, L., et al. (2005) Nature 435, 828-833; and Lu, J., et al. (2005) Nature 435:834-838).
There is also strong evidence that miRNAs play a role in mammalian hematopoiesis. In mice, miR-181, miR-223 and miR-142 are differentially expressed in hematopoietic tissues, and their expression is regulated during hematopoiesis and lineage commitment (Chen, C. Z., et al. (2004) Science 303, 83-86). The ectopic expression of miR-181 in murine hematopoietic progenitor cells led to proliferation in the B-cell compartment (Chen, C. Z., et al. (2004) Science 303, 83-86). Systematic miRNA gene profiling in cells of the murine hematopoietic system revealed different miRNA expression patterns in the hematopoietic system compared with neuronal tissues, and identified individual miRNA expression changes that occur during cell differentiation (Monticelli, S., et al. (2005) Genome Biology 6, R71). A recent study has identified down-modulation of miR-221 and miR-222 in human erythropoietic cultures of CD34+ cord blood progenitor cells (Felli, N., et al. (2005) Proc. Natl. Acad. Sci. USA. 102, 18081-18086). These miRNAs were found to target the oncogene c-Kit. Further functional studies indicated that the decline of these two miRNAs in erythropoietic cultures unblocks Kit protein production at the translational level leading to expansion of early erythroid cells (Felli, N., et al. (2005) Proc. Natl. Acad. Sci. USA. 102, 18081-18086). In line with the hypothesis of miRNAs regulating cell differentiation, miR-223 was found to be a key member of a regulatory circuit involving C/EBPa and NH-A, which controls granulocytic differentiation in all-trans retinoic acid-treated acute promyelocytic leukemic cell lines (Fazi, F., et al. (2005) Cell 123, 819-831).
miRNAs have also been found deregulated in hematopoietic malignancies. Indeed, the first report linking miRNAs and cancer involved the deletion and down regulation of the miR-15a and miR-16-1 cluster, located at chromosome 13q14.3, a commonly-deleted region in chronic lymphocytic leukemia (Calin, G. A, et al. (2002) Proc. Natl. Acad. Sci. USA 99, 1554-15529). High expression of miR-155 and host gene BIC was also reported in B-cell lymphomas (Metzler M., et al. (2004) Genes Chromosomes and Cancer 39; 167-169). More recently it was shown that the miR-17-92 cluster, which is located in a genomic region of amplification in lymphomas, is overexpressed in human B-cell lymphomas and the enforced expression of this cluster acted in concert with c-MYC expression to accelerate tumor development in a mouse B cell lymphoma model (He, L., et al. (2005) Nature 435, 828-833). These observations indicate that miRNAs are important regulators of hematopoiesis and can be involved in malignant transformation.
Platelets play an essential role in hemostasis and thrombosis. They are produced from in large numbers from their parent cells, bone marrow megakaryocytes, and arise from fragmentation of the cytoplasm. Only recently has the molecular basis of what may turn out to be a large family of related disorders affecting platelet production started to be defined. If the level of circulating platelets drops below a certain number (thrombocytopenia), the patient runs the risk of catastrophic hemorrhage. Patients with cancer who have received chemotherapy or bone marrow transplants usually have thrombocytopenia, and the slow recovery of platelet count in these patients has been a concern. The demand for platelet units for transfusion has been steadily increasing primarily because of the need to maintain a certain platelet level in such patients with cancer or those undergoing major cardiac surgery.
Identification of microRNAs that are differentially-expressed in cancer cells (e.g., leukemia cells) may help pinpoint specific miRNAs that are involved in cancer and other disorders (e.g., platelet disorders). Furthermore, the identification of putative targets of these miRNAs may help to unravel their pathogenic role. In particular, discovering the patterns and sequence of miRNA expression during hematopoietic differentiation may provide insights about the functional roles of these tiny non-coding genes in normal and malignant hematopoiesis.
There is a need for novel methods and compositions for the diagnosis, prognosis and treatment of cancer, myeloproliferative disorders and platelet disorders (e.g., inherited platelet disorders).